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Using DevTools Features Without Opening DevTools

I commonly see questions along the lines of "I really like feature X of DevTools, but it
stops working when I close DevTools. How do I keep feature X running even when DevTools
is closed?"

The short answer is: you probably can't.

However, you can hack together a Puppeteer script that launches
Chromium, opens a remote debugging client, then turns on the DevTools feature that you like
(via the Chrome DevTools Protocol), without ever explicitly opening DevTools.

For example, the script below lets me overlay the FPS Meter over the top-right of the
viewport, even though DevTools never opens, as you can see in the video below.